Allylic pyrophosphates play a central role in isoprenoid biosynthesis. We plan to study the isomerization and cyclization of two key substrates, farnesyl and geranyl pyrophosphate, which are believed to be the universal precursors of sesquiterpenes and monoterpenes, respectively. Using cell-free fungal extracts we will study the enzymatic conversion of farnesyl pyrophosphate to the sesquiterpene hydrocarbon trichodiene, catalyzed by Trichodiene synthetase. The use of appropriately labeled precursors will allow us to define the conformation of the cyclizing precursor and establish the role of the pyrophosphate counter ion. We will extend our previous studies of the enzymatic conversion of farnesyl to nerolidyl pyrophosphate by pu rifying both the relevant enzyme, farnesyl pyrophosphate isomerase, and trichodiene sythetase and comparing the mechanism of action of these two related enzymes. In collaboration with Professor Rodney croteauj of Washington State University we will test a stereochemical model of monoterpene biosythesis using the plant enzymes, bornyl pyrophosphate synthetase and fenchol synthetase. It is expected that the information gained from these studies will be applicable to an understanding of the catalysis of biological carbon-carbon bond formation in general.